Deposition of metal-rich mine tailings, metal smelting, leather tanning, electroplating, emissions from gas exhausts, energy and fuel production, downwash from powerlines, intensive agriculture and sludge dumping are the most important human activities which contaminate soil systems with large amounts of toxic metals. The list of sites contaminated with toxic metals grows larger every year, presenting a serious health problem and a formidable danger to the environment. In spite of the growing number of metal-contaminated soil sites, the costly process of removing and burying metal-contaminated soils, or isolating the contaminated sites, remain the most commonly used methods for reclaiming metal-contaminated soils.
Moreover, many heavy metals exist in situ in their anionic form and some of these metals may pose unique remediation problems. For instance, chromium exists in soil in two different oxidation states; cationic Cr.sup.+3 Cr(III)! and its oxidized, hexavalent anionic form Cr.sup.+6 Cr(VI)!. Both species of chromium have very different properties. Reduced cationic Cr(III) is insoluble in soils, and is therefore unable to move into the food chain. Because it is not readily available to be included in the food chain and thus its toxicity is inherently low, reduced Cr(III) posses only limited health risks.
On the other hand, anionic chromium (VI) is readily leachable and is mobile in soil and may be taken up by plants or released into the groundwater. It can move much more easily into the food chain than its reduced species and is capable of producing toxicity in humans and other animals. A dramatic reduction in the toxicity of Cr(VI) in soil could be achieved by finding a way to efficiently convert Cr(VI) to Cr(III).